Remove `MultiheadAttention` weights from constants (#39768)
Summary:
The weights of the `MultiheadAttention` were incorrectly listed as constants, which produced warnings when converting to a TorchScript module.
```py
import torch
import torch.nn as nn
multihead_attn = nn.MultiheadAttention(256, 4)
torch.jit.script(multihead_attn)
```
Warnings:
```
/home/michael/.local/lib/python3.8/site-packages/torch/jit/_recursive.py:151: UserWarning: 'q_proj_weight' was found in ScriptModule constants, but it is a non-constant parameter. Consider removing it.
warnings.warn("'{}' was found in ScriptModule constants, "
/home/michael/.local/lib/python3.8/site-packages/torch/jit/_recursive.py:151: UserWarning: 'k_proj_weight' was found in ScriptModule constants, but it is a non-constant parameter. Consider removing it.
warnings.warn("'{}' was found in ScriptModule constants, "
/home/michael/.local/lib/python3.8/site-packages/torch/jit/_recursive.py:151: UserWarning: 'v_proj_weight' was found in ScriptModule constants, but it is a non-constant parameter. Consider removing it.
warnings.warn("'{}' was found in ScriptModule constants, "
/home/michael/.local/lib/python3.8/site-packages/torch/jit/_recursive.py:151: UserWarning: 'in_proj_weight' was found in ScriptModule constants, but it is a non-constant parameter. Consider removing it.
warnings.warn("'{}' was found in ScriptModule constants, "
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39768
Reviewed By: zhangguanheng66
Differential Revision: D21977032
Pulled By: ngimel
fbshipit-source-id: c2c3d0605a51324a9541f5a2caca7ab7a518dc00
